[PATCH] Fix runtime linker auditing on aarch64

[Carlos O'Donell]

On 9/22/20 9:16 PM, Ben Woodard via Libc-alpha wrote:
> The dynamic linker's auditing was not working on aarch64. See PR#26643
> https://sourceware.org/bugzilla/show_bug.cgi?id=26643
> 
> There were two distinct problems:
>   * _dl_runtime_resolve was not preserving x8 the indirect result location
>     register.
>   * The NEON Q registers pushed onto the stack by _dl_runtime_resolve
>     were twice the size of D registers extracted from the stack frame by
>     _dl_runtime_profile.
> 
> To fix this
>   * The La_aarch64_regs structure was expanded to include x8 and the full
>     sized NEON V registers that are required to be preserved by the ABI.
>   * _dl_runtime_profile needed to extract registers saved by
>     _dl_runtime_resolve and put them into the new correctly sized
>     La_aarch64_regs structure.
>   * The return value structure La_aarch64_retval also didn't have the correctly
>     sized NEON V registers.
> 
> As a couple of additional cleanups
>   * The names of the NEON registers saved within the La_aarch64_regs and the
>     La_aarch_retval structures referred to the old D registers which were
>     doubles. Now the registers are quads and are called V for vector registers.
>     So the name of the field in the structure and the names of the offsets
>     within that structure were named to use the more modern names.
>   * The ABI specification says that r0-r7 + r8 the indirect result location
>     register as well as the NEON v0-v7 registers can be used to return values
>     from a function. Therefore, I addded those to the La_aarch64_retval
>     structure so that it also correctly matches the ABI.
> 
> An additional problem not addressed by this patch is what to do about the
> changes to the aarch64 ABI needed to support SVE. A discussion about what to
> do about that was begun on libc-alpha here:
> https://sourceware.org/pipermail/libc-alpha/2020-September/117797.html
> ---
>  sysdeps/aarch64/bits/link.h     | 17 ++++----
>  sysdeps/aarch64/dl-link.sym     |  4 +-
>  sysdeps/aarch64/dl-trampoline.S | 75 +++++++++++++++++++++------------
>  3 files changed, 59 insertions(+), 37 deletions(-)
> 
> diff --git a/sysdeps/aarch64/bits/link.h b/sysdeps/aarch64/bits/link.h
> index 0c54e6ea7b..2b43ace57c 100644
> --- a/sysdeps/aarch64/bits/link.h
> +++ b/sysdeps/aarch64/bits/link.h
> @@ -23,19 +23,20 @@
>  /* Registers for entry into PLT on AArch64.  */
>  typedef struct La_aarch64_regs
>  {
> -  uint64_t lr_xreg[8];
> -  uint64_t lr_dreg[8];
> -  uint64_t lr_sp;
> -  uint64_t lr_lr;
> +  uint64_t    lr_xreg[9];
> +  __uint128_t lr_vreg[8];
> +  uint64_t    lr_sp;
> +  uint64_t    lr_lr;

This breaks ABI and does not address what to do about SVE.

If you argue that LD_AUDIT was always broken for AArch64 then
you get away with breaking ABI *once* and that one time you
break it to fix the ABI should include all the currently known
breakages that are out there.

I think we'll need feedback from Arm on this to get their input
on the direction to take here.

Granted we always have LAV_CURRENT we can bump to change the
interface, but that's a lot of code to write to handle that
and it would require struct-copying to support a newer larger
sized structure.

-- 
Cheers,
Carlos.